During exercise, energy turnover increases and adrenergic mechanisms play an important role in this regulation. In addition, increased adrenergic activity during exercise also results in an increased heart rate and in an enhanced force of myocardial contraction as well as in vasoconstriction in the splanchnic circulation, in the kidneys, and in noncontracting muscles. These circulatory changes favor a redistribution of blood flow to exercising muscle as well as an increased cardiac output (Rowell. Human circulation regulation during physical stress. Oxford University Press, New York, 1986). Furthermore, the adrenergic activity stimulates sweat glands and thereby influences thermoregulation, and it causes an increased contractility of skeletal muscle as well as influences exercise-induced suppression of components of the human immune system. In the present chapter, it is demonstrated how adrenergic activity can influence substrate mobilization and utilization both directly and indirectly via secretion of hormones.